LEAD Action News Vol
1 no 2 Winter 1993 ISSN 1324-6011
Incorporating Lead Aware Times ( ISSN 1440-4966) and Lead Advisory Service News ( ISSN 1440-0561)
The journal of The LEAD (Lead Education and Abatement Design) Group Inc.

This article is extracted from the interim report ("Revising
Australian Guidelines for Lead", July 1993) to the NHMRC, of the
RMIT (Royal Melbourne Institute of Technology) consultancy team, for
which Deni Greene is the senior researcher. The final report is due out
at the end of July '93.

Lead moves into and throughout ecosystems. Atmospheric lead is
deposited in vegetation, ground and water surfaces. The chemical and
physical properties of lead and the biogeochemical processes within
ecosystems will influence the movement of lead through ecosystems. The
metal can affect all components of the environment and can move through
the ecosystem until it reaches an equilibrium. Lead accumulates in the
environment, but in certain chemical environ­ments it will be
transformed in such a way as to increase its solubility (e.g., the
formations of lead sulfate in soils), its bioavailability or its
toxicity. The effects of lead at the ecosystem level are usually seen as
a form of stress (US EPA 1986).

In general, there are three known ways in which lead can adversely
affect ecosystems. Populations of micro-organisms may be wiped out at
soil lead concentrations of 1,000 parts per million (ppm) or more,
slowing the rate of decomposition of matter. Populations of plants,
micro-organisms and inverte­brates may be affected by lead
concentrations of 500 to 1,000 ppm, allowing more lead-tolerant
populations of the same or different species to take their place. This
will change the type of ecosystem present. At all am­bient atmospheric
concentrations of lead, the addition of lead to vegetation and animal
surfaces can prevent the normal bio­chemical process that purifies and
repurifies the calcium pool in grazing animals and decomposer organisms
(UNEP 1991).

Exposure routes for lead to the environment

The main sources of lead entering an ecosystem are atmospheric lead
(primarily from automobile emissions), paint chips, used ammunition,
fertilisers and pesticides and lead-acid batteries or other industrial
products. The transport and distribution of lead from major emission
sources, both fixed and mobile, are mainly through air (UNEP 1991).
While most of the lead discharged into air falls out near the source,
about 20 percent is widely dispersed. Studies have demonstrated that
measurements of lead in Greenland rose and fell with the rise and
decline of use of alkyl-leaded petrol in the United States Eurasia and
Canada over the past century (Isotopic
evidence for the source of lead in Greenland snows since the late 1960s;
K. J. R. Rosman, W. Chisholm, C. F. Boutron, J. P. Candelone & U. Görlach;
Nature 362, 333 - 335; 25 March 1993). The size of the lead
particles will govern how far they move from the source.

Effects of lead on soil

It is known that lead accumulates in the soil, particularly soil with
a high organic content (US EPA 1986). Lead deposited on the ground is
transferred to the upper layers of the soil surface, where it may be
retained for many years (up to 2000 years). In undisturbed ecosystems,
organic matter in the upper layer of soil surface retains atmospheric
lead. In cultivated soils, this lead is mixed with soil to a depth of
25C1ll (i.e., within the root zone). Atmospheric lead in the soil will
continue to move into the micro-organism and grazing food chains, until
an equilibrium is reached.

Given the chemistry of lead in soil, the US EPA (1986) suggests that
the uneven distribution of lead in ecosystems can displace other metals
from the binding sites on the organic matter. It may hinder the chem­ical
breakdown of inorganic soil fragments and lead in the soil may become
more soluble, thus being more readily available to be taken up by
plants.

Effects of lead on plants

Plants on land tend to absorb lead from the soil and retain most of
this in their roots. There is some evidence that plant foliage may also
take up lead (and it is possible that this lead is moved to other parts
of the plant). The uptake of lead by the roots of the plant may be
reduced with the application of calcium and phosphorus to the soil. Some
species of plant have the capacity to accumulate high concentrations of
lead (UNEP, WHO and ILO 1991).

The pores in a plant's leaves let in carbon dioxide needed for
photosynthesis and emit oxygen. Lead pollution coats the surface of the
leaf and reduces the amount of light reaching it. This results in
stunting the growth or killing the plants by reducing the rate of
photosynthesis, inhibiting respiration, encouraging an elongation of
plant cells influencing root development 0; by causing pre-mature aging.
Some evidence suggests that lead can affect population genetics. All
these effects have been observed in isolated cells or in hydroponically
grown plants in solutions of around 1-2 ppm of lead in soil moisture
e.g., the lead levels experienced by ecosystems near smelters or
roadsides).

Lead in air may be transferred to plants directly through fallout or
indirectly through up-take from the soil. The pattern and degree of lead
accumulation are largely influenced by the state of growth of the
vegetation; i.e., active growth periods in spring as compared to low
growth periods through autumn and winter.

Effects of lead on micro-organisms

Evidence exists to show that lead at the concentrations occasionally
found near roadsides (i.e., 10,000 - 40,000 ppm dry weight), can wipe
out populations of bacteria and fungi on leaf surfaces and in soil. This
can have a significant impact, given that many of these micro-organisms
are an essential part of the decomposing food chain. The micro-organism
populations affected are likely to be replaced by others of the same or
different species, although these may be less efficient at decomposing
organic matter. Evidence also suggests that micro-organisms can make
lead more soluble and hence more easily absorbed by plants. That is,
bacteria exude organic acids that lower the pH in the immediate vicinity
of the plant root.

Effects of lead on animals

Lead affects the central nervous system of animals and inhibits their
ability to synthesize red blood cells. Lead blood concentrations of
above 40 µg/dl can produce observable clinical symptoms in domestic
animals. Calcium and phosphorus can reduce the intestinal absorption of
lead (US EPA 1986). The US EPA report generalizes that a regular diet of
2-8 mg of lead per kilogram of body weight per day, over an extended
period of time, will cause death in most animals. Grazing animals are
directly affected by the consumption of forage and feed contaminated by
airborne lead and somewhat indirectly by the up-take of lead through
plant roots. Invertebrates may also accumulate lead at levels toxic to
their predators.

Lead shot and lead weight can severely affect individual organisms
and threaten ecosys­tems (WHO 1989). After three to ten days of
waterfowl ingesting lead shot, the poison will reach the bloodstream and
be carried to major organs, like the heart, liver and kidneys. By the
17th to 21st day the bird falls into a coma and dies. Following the
ingestion of lead shot, lead toxicosis has been observed in Magpie
geese, Black swans, several species of duck (including Black duck and
Musk duck) and Hardhead species (OECD 1993).Organic lead is much more
readily taken up by birds and fish (WHO 1989). Aquatic organisms take up
inorganic lead through a transfer of lead from water and sediments; this
is a relatively slow process. Organic lead is rapidly taken up by
aquatic organisms from water and sediment. Aquatic animals are affected
by lead at water concentrations lower than previously thought safe for
wildlife. These concentrations occur often, but the impact of
atmospheric lead on specific sites with high aquatic lead levels is not
clear (US EPA 1986).